Bioburden Testing and Sterility Testing

Producing safe products is a core goal of all medical device manufacturers and sterility assurance is a key component in achieving that goal.  Many single use medical devices are terminally sterilized by ethylene oxide or radiation methods such as gamma or e-beam.  The FDA and other regulatory bodies requires that the sterilization process be validated and these validations typically require a bioburden and sterility testing.  Sterility testing and bioburden testing are also performed on devices as part of routine quality control.

Bioburden Testing of Medical Devices

A bioburden test determines how many microbes are on the medical device.  Before performing the test, a suitability test must first be performed.   The purpose of the bioburden suitability test (also called method validation) is to ensure that the bioburden test method is effective in recovering microorganisms that are present on the device and to show that the test method will allow the growth of the device microorganisms.  To perform the method validation, a known low number of microorganisms are placed on a sterile device and then removed using the same method that would be used for the actual bioburden test. Based on the outcome of the method validation, a recovery factor is determined to account for the percentage of microorganisms that were not able to be removed from the device.

After an acceptable method validation is complete, the device will undergo bioburden testing to determine their microbiological load. This device bioburden level is often used in sterilization validations to calculate the verification or sterilization dose a device may need. Additionally, as part of quality control, quarterly bioburden monitoring is done to determine whether the microbiological load on a device has changed.

Sterility Testing of Medical Devices

When performing a sterility test, it is imperative that the lab is testing that the device is actually sterile rather than leaching out chemicals that could impair the growth, or even kill, microorganisms.  Therefore, just like bioburden testing, sterility testing also has a suitability test.  The sterility suitability test is called bacteriostasis and fungistasis testing. A bacteriostasis and fungistasis (B&F) test measures whether a device inhibits the growth of bacteria or fungi, which may lead to false negatives during sterility testing.  The test is performed by adding the device to media and then inoculating the media with less than 100 CFU of three different organisms.  If the three organisms are able to turn the media turbid, the device passes the B&F test and sterility testing can begin.

Sterility testing of medical devices are typically performed using media called SCDM (Soybean-Casein Digest Medium).  The device is physically placed in the media.  The amount of sample required depends on the testing method chosen, but all methods require an incubation time of fourteen days.  If the media has turned turbid after fourteen days, the sample has failed the sterility test.

ISO 11137 Dose Audits

Verification dose and dose audits are performed according to ANSI/AAMI/ISO 11137. Dose audits are done as a check to ensure that the sterilization process is still producing adequately sterilized products. All sterility testing is conducted in an ISO Class 5 environment.

The microbiology lab at Pacific BioLabs is experienced in preparing and testing a wide variety of devices, from small needles to large, complex devices. For large devices, Sample Item Portion (SIP) preparation may be necessary. This consists of cutting a device into pieces for testing, or selecting the most appropriate parts of a device for testing.

Case Study: Challenges in Bioburden Testing

Potential contaminants such as those from personnel, equipment, raw materials and even the air in a manufacturing facility can affect the quality of medical device and pharmaceutical products. Therefore, manufacturers require bioburden and sterility tests to assure these products are manufactured to utmost quality standards.


PBL has been supporting its clients with bioburden and sterility testing for many years. Our microbiology department recently worked with a client that produces a range of high-purity pigments for pharmaceutical tablet coatings, gelatin capsules, nutraceuticals, liquid suspensions, and contrast agents. The client asked PBL to determine the enumeration and microbial characterization of viable microorganism populations in their product so they could determine the effective sterilization dose.


Our client’s product was a powder that was a strong acid and difficult to neutralize. Our microbiology team faced the challenge of neutralizing the antimicrobial properties of this powder. To ensure that the number of microorganisms present in the powder could be determined, PBL researched different method development techniques for the powder and associated bottle and cap.

Our Methodology

A successful method validation should be effective at recovering these microorganisms while not inhibiting the growth of the recovered organisms. Inadequate microorganism recovery or active antimicrobial properties can result in false negatives. Therefore it is important to perform method validation to ensure the test method’s efficiency in recovering microorganisms present in the product. Prior to method validation, our microbiology team performed method development with various neutralization methods, media types, filter membranes, and shaking & sonication techniques for the client’s powder, bottle, and cap. These led to the development of operating procedures that were valid for use on these products.

Our Solution

As a result of our microbiology team’s expertise and efforts in method development, PBL was able to accurately determine the population of viable microbes in the client’s powder, bottle and cap and consequently determine an effective verification dose. We are proud to have contributed to this challenging yet interesting bioburden study.

Bioburden and Sterility Testing Services

Read More About Bioburden and Sterility Testing

PBL’s Sterility Assurance Booklet
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